1. Field of the Invention
The present invention relates to an epitaxial wafer and a method of preparing the same, and more particularly, it relates to an epitaxial wafer for growing a GaN thin film thereon to be applied to a blue light emitting device or various devices for the ultraviolet region, and a method of preparing the same.
2. Description of the Background Art
FIG. 13 is a sectional view showing the structure of an epitaxial wafer employing a sapphire substrate to be applied to a GaN blue light emitting device (LED), which is now on the market.
Referring to FIG. 13, this epitaxial wafer comprises a sapphire substrate 11, a gallium nitride (GaN) buffer layer 12 which is formed on the substrate 11, and a hexagonal GaN epitaxial layer 13 which is formed on the GaN buffer layer 12. In this epitaxial wafer, the GaN buffer layer 12 is adapted to relax distortion resulting from the difference between the lattice constants of the sapphire substrate 11 and the GaN epitaxial layer 13.
FIG. 14 is a sectional view showing the structure of a GaN blue light emitting device employing the epitaxial wafer shown in FIG. 13.
Referring to FIG. 14, a cladding layer 14, a active layer 15, another cladding layer 16 and a GaN epitaxial layer 17 are successively formed on the epitaxial wafer shown in FIG. 13 in this blue light emitting device, while ohmic electrodes 18 and 19 are formed on the GaN epitaxial layers 13 and 17 respectively.
Referring to FIGS. 13 and 14, the substrate 11 of this epitaxial wafer is prepared from insulating sapphire. In order to prepare the device by forming the electrodes, therefore, photolithographic patterning must be performed at least twice while the nitride layers must be etched by reactive ion etching, which complicated process steps. Further, sapphire is hard to treat due to its hardness. In addition, sapphire is uncleavable and thus has such a problem in application to a light emitting device that it cannot be applied to a laser diode utilizing a cleavage plane as an optical resonator.
To this end, an attempt has been made for employing a conductive volatile material such as GaAs for the substrate in place of sapphire having the aforementioned disadvantages.
When the volatile material such as GaAs is employed as the substrate, however, volatile arsenic (As) is lost from the back surface of the substrate in epitaxial growth, to disadvantageously damage the substrate.
Consequently, the treatment temperature cannot be much increased in growth of a GaN epitaxial layer. Thus, the obtained GaN epitaxial layer is limited in improvement of characteristics.